1. Field of the Invention
The present invention relates to a photomask for use in transferring a predetermined rectangular pattern to a resist film through of a lithography technique.
2. Description of the Background Art
A lithography technique is usually used for forming a circuit pattern in the process of manufacturing a semiconductor device. The technique fundamentally comprises a coating step for coating a substrate such as wafer with photoresist, an exposure step for impinging light from a suitable light source (e.g., light in an ultraviolet region) upon a photomask having a predetermined pattern to transfer the pattern to the photoresist, and a development step for developing the photoresist to provide the photoresist having the predetermined pattern.
FIG. 10 is a perspective view of the major part of a conventional photomask 70 for use in the exposure step of the lithography. In FIG. 10, reference numeral 71 designates a glass substrate, on which a light shielding layer 72 made of chromium and the like is formed. The light shielding layer 72 is provided with a main pattern 73 which is, for example, rectangular. The main pattern 73 is formed in the shape similar to the pattern to be transferred to the photoresist. The term "rectangular" means a shape including a square and an oblong.
FIG. 11 is a schematic view of an exposing device. The exposing device comprises a light source 81 for emitting light in the ultraviolet region downwardly. Light L from the light source 81 is directed through a lens 82 to the photomask 70 of FIG. 10. Some of the incident light L passing through the rectangular main pattern 73 is introduced to a photoresist surface 84 through a lens 83. The light L incident on the light shielding layer 72 is shielded by the light shielding layer 72. Thus, a pattern corresponding to the main pattern 73 (hereinafter referred to as a "transferred pattern") is transferred to the photoresist surface 84.
FIG. 12 is a graph showing light intensity distribution on the photoresist surface 84. The graph of FIG. 12 was derived from the main pattern 73, 0.4 .mu.m square. The abscissa denotes a distance from a point O, being the center of the transferred pattern, in the X direction (of FIG. 10), while the ordinate denotes a relative light intensity where the intensity at whole image exposure is taken as "1". The term "whole image exposure" refers to the exposure of the photoresist surface 84 through a photomask formed of the glass substrate 71 only.
A profile of the light intensity around the main peak shows Gaussian type distribution. That is, the light intensity is decreased gradually and continuously as the distance from the point O is increased. In particular, when the light intensity is low at the point O and varies moderately, the shape of the rectangular pattern transferred to the photoresist surface 84 is not sharp at the edges thereof. As a result, it is difficult to reproduce the transferred pattern corresponding to the main pattern 73 to the photoresist surface 84 faithfully. A photomask having only a main pattern for a contact hole conventionally has the above-mentioned problem.